Hi Just Jack,
April is a good month to do the Catalan Way from Barcelona, but certain aspects must be taken into account:
From Barcelona to the Monastery of Montserrat there are hardly any hostels, only hotels and there are many, very few pilgrims.
From Montserrat you have good hostel infrastructure. Once in Tàrrega I would go through Huesca and San Juan de la Peña to Santa Cilia, from here is the French Way through Aragon. I recommend it since the hostels are almost insured, the landscape and the historical places it crosses are spectacular.
The weather in April can range from cold to hot, from rainy to dry, so it is unpredictable, but in general terms it is usually a good month for walking, since temperatures during walking hours are usually optimal.
Regarding the number of pilgrims: it may be lonely and you will not see any more pilgrims or you may meet four or five, it is also unpredictable. In any case there will be very few.
I have made the path till Tàrrega, I plan to continue it in the spring. Maybe in April too.
Attention!
Easter is high season for all accommodations. This year it goes from Palm Day, Sunday 13 to Easter Monday, April 21
Bon Camí.